In a typical internal-combustion engine a camshaft controls the timing of the valves relative to the movement of the corresponding pistons. A distributor in turn controls the actuation of spark plugs, which introduce electrical sparks into the combustion chambers of the pistons to ignite an air/fuel mixture to drive the pistons. The true ignition point of the air/fuel mixture is marked by a combustion process called flaggregation (i.e., at the instant flaggregation occurs, true ignition begins).
The flaggregation of each cylinder in an internal-combustion engine is typically not maintained at a constant rate, and can vary anywhere within the range of approximately 20 to 60 meters/second. The faster the rate of flaggregation, the more easily combustion can take place during a typical engine cycle. One reason for slow flaggregation, however, is that the probability of chemical combustion around the spark plug at the instant that a spark occurs is relatively poor. Accordingly, there is a great degree of variability in flaggregation among internal-combustion engines.
In tests of internal-combustion engines, the same cylinder typically does not produce the same pressure in consecutive firings. One reason for this pressure variation is that the probability of combustion in two firings is almost never the same. In order for true ignition to occur, the fuel and oxygen molecules in a combustion chamber must reach a threshold state of vibrational excitation and then burn with sufficient energy to maintain a self-sustaining combustion. In current internal-combustion engines this threshold state of vibrational excitation is obtained during the compression stroke of the piston. It is this group of molecules that reach the threshold level of vibrational excitation which produce the resulting self-sustaining burn to carry out the combustion process. However, because these excited states are obtained by thermal collisions of the molecules of the air-fuel mixture, the distribution of excited states of the molecules generally follows a Boltzmann distribution. Thus, at the initiation of ignition, it is believed that typically less than approximately 30% of the available combustible molecules reach the threshold state of vibrational excitation. Accordingly, it is also believed that flaggregation occurs relatively slowly at least approximately 70% of the time in typical internal-combustion engines.
One method of increasing the rate of flaggregation is to increase both the octane of the fuel and the compression ratio of the engine. By increasing the compression ratio of the engine, the probability that the various molecules of the air/fuel mixture will reach the threshold level of vibrational excitation is increased. However, because air contains nitrogen, various NO.sub.x compounds are formed under such high compression. Accordingly, high-octane/high-compression engines have been eliminated for all practical purposes for failure to meet pollution standards because of the formation of NO.sub.x.
In an attempt to make low-compression engines burn more efficiently, multiple-spark discharges have been used. Although multiple-spark discharges have been known to better facilitate the burn efficiency of high-compression engines, such modifications can also cause low-compression engines to achieve flaggregation rates similar to high-compression engines. However, only the initial spark, i.e., the spark at the front of the flame, requires the multiple discharge, and it is difficult to control such a system so that the multiple discharge occurs in this way. Also, because of the energy distribution of the electrons in the spark discharges, most of the electrons do not excite the molecules of the air-fuel mixture to the threshold level of vibrational excitation. Accordingly, this is a relatively inefficient means for increasing the rate of flaggregation.